Lack of specific information about the specific organization of the contractile proteins in smooth muscles makes it difficult to determine whether mechanisms of contraction proposed for skeletal muscles can be applied to smooth muscles. The major aim of the proposed work is to determine the precise localization of myosin, actin, and tropomyosin and any changes in localization that occur during contraction so that the applicability of the sliding filament mechanism may be tested. Fluorescent antibodies, polarization microscopy and electron microscopy will be used to gain the necessary information. An additional hypothesis will be tested; that there exist specific fiber types within smooth muscles which may be recognized by immunologic differences in contractile proteins. Antibodies from different muscles, and affinities for different smooth muscles will be used to test the hypothesis. The ability to recognize specific fiber types would be a significant tool in studying pathological processes involving smooth muscles.